Short answer
In a two-wire loop, the same current travels through the supply, transmitter, cable, and input load. The transmitter uses part of the available voltage to operate while controlling the current that represents the measured value.
What to look for
The loop must leave enough voltage at the transmitter at the highest required current. Cable resistance, barriers, isolators, and input resistance all consume part of the budget.
A loop that works at room temperature but fails at the top of the range may be running out of voltage headroom. Measure the voltage at the transmitter, not only at the power supply.
When the symptom appears in the plant
A pressure transmitter loop included the field device, a local digital indicator, and the PLC input card. The PLC constantly reported a 'Wire Break' error, but the transmitter seemed to function when connected directly to a calibrator. The technician calculated the total loop resistance and found it exceeded the 600-ohm limit of the 24V power supply. The local indicator added too much burden. Upgrading to a higher compliance voltage supply solved the problem.
Five checks before replacement
- Calculate the total loop resistance by summing the resistance of the wiring, the PLC input card (typically 250 ohms), and any other loop devices.
- Determine the maximum allowable loop resistance based on the power supply voltage (e.g., for a 24V supply and 12V transmitter minimum, (24-12)/0.02 = 600 ohms max).
- Measure the voltage across the transmitter terminals while forcing a 20mA signal; it must remain above the manufacturer's specified minimum operating voltage.
- Verify the power supply capacity; it must provide enough current for all loops on the circuit simultaneously (assume 20mA per loop).
- Check for 'hidden' resistance sources, such as corroded terminals, intrinsic safety barriers, or very long cable runs of thin gauge wire.
Always check the burden voltage of passive loop displays or chart recorders; they often drop 3-5 volts from the loop.
A 250-ohm precision resistor is commonly used at the PLC to convert a 4-20mA current loop into a 1-5V voltage signal.
When using intrinsically safe barriers, remember they add significant resistance to the loop.
Bench checklist
- Confirm the supply at the device terminals.
- Confirm the meter is in the correct mode and position.
- Confirm the input range matches the signal type.
- Confirm the reference/common is intentional.
- Record the reading before changing the wiring.
Reference families
- ISA-50.1 Standard
- Instrument Loop Design Guidelines
Always verify the current edition and the exact device manual for the installation.
Official references
These links point to standards bodies, industry organizations, or manufacturer documentation. Standards landing pages may require purchase or institutional access.
- IEC 60381-1: Analogue direct-current signals IEC
Official standard landing page for analog DC signals in process-control systems.
- 4 to 20 mA Current Loop Configurations BAPI
Manufacturer application note with two- and three-wire loop-power arrangements.
- What Is a 4-20 mA Current Loop? Fluke
Explains supply, voltage drops, resistance, and loop power budget.
- AN-1312: 4 mA to 20 mA DC Measurements Analog Devices
Application note covering summed voltage drops and loop-powered transducers.